Blender 2.80: Delete Collection or clear the intial scene in scripting mode
Unlink all children from the scene collection

Not a fan of using outliner operators in scripts. Unlinking or removing the collections from a scene will also have the same effect of clearing a scene.

Script to unlink all the child collections from the scene's master collection.

import bpy
context = bpy.context
scene = context.scene

for c in scene.collection.children:
    scene.collection.children.unlink(c)


Theoretically could remove the child collections using bpy.data.collections.remove(c) above instead of unlink, however I found this has the tendency to seg fault my build of 2.8._ (See EDIT below)

Running this after removes the orphan collection

import bpy

for c in bpy.data.collections:
    if not c.users:
        bpy.data.collections.remove(c)


Similarly could remove the collections objects from blender file with 

for o in c.objects:
    bpy.data.objects.remove(o)


EDIT. Update: removing collections from bpy data

import bpy

name = "Collection 1"
remove_collection_objects = True

coll = bpy.data.collections.get(name)

if coll:
    if remove_collection_objects:
        obs = [o for o in coll.objects if o.users == 1]
        while obs:
            bpy.data.objects.remove(obs.pop())

    bpy.data.collections.remove(coll)